The present invention relates generally to the field of compressors and specifically to preventing cross-contamination of the diverse fluid mediums present in a piston driven pressure intensifier.
An overriding concern in the compressor field is product gas (i.e., the gas compressed by the compressor) contamination from the intermixing of the product gas with operating fluids (e.g., hydraulic fluids or other compressor fluids) during the compression process. Product gas contamination is particularly problematic in semiconductor processing applications such as isostatic pressing processes which require high purity compressed gases and pressure levels of approximately 1000 atmospheres. In order to achieve such high pressures, a number of hydraulic piston driven compressors are interconnected so as to provide staged pressure increases. Staged pressure increases allow a gas to be pressurized without a substantial increase in gas discharge temperature (e.g., by using inter-stage coolers to cool the gas between stages). Specifically a compressed gas output from a first hydraulic piston driven compressor (i.e., a first "stage") passes through an inter-stage cooler and is input to the next hydraulic piston driven compressor (i.e., the second stage) where it is further compressed, cooled and passed to the next stage, and so on. In this manner gas pressure increases gradually and exceedingly high gas discharge temperatures are avoided.
In order to understand how the present invention reduces product gas contamination, it is first necessary to understand how conventional piston-type compressors increase gas pressure. With this understanding, the problems which cause product gas contamination in conventional compressors will be apparent.
Referring to FIG. 1, one stage (i.e., one compressor 11) of a conventional piston-type multi-stage compressor is depicted in section. Each compressor 11 typically comprises a hydraulic chamber 13 containing hydraulic fluid (e.g., oil) and a hydraulic piston 15, and a gas chamber 17 that includes an inlet 19 for receiving the gas to be compressed (i.e., product gas) and an outlet 21 for supplying compressed product gas to a subsequent stage or to a standard processing chamber. The gas chamber 17 further includes a gas piston 23 operatively coupled to the hydraulic piston 15 by a piston rod 25 that extends through and is slidably mounted in a bore 27 (hereinafter "hydraulic bore 27") in the hydraulic chamber 13 and a bore 29 (hereinafter "gas bore 29") in the gas chamber 17.
In operation, a motor (not shown) operatively coupled to the piston rod 25 moves the piston rod 25 back and forth. When the piston rod 25 moves toward the hydraulic chamber 13 (i.e., during a frontstroke) product gas is drawn into the gas chamber 17 via the inlet 19; as the piston rod 25 moves toward the gas chamber 17 (i.e., during a backstroke) product gas is compressed and, after a desired pressure is obtained, the compressed product gas is exhausted from the gas chamber 17 via the outlet 21. Typically the hydraulic piston 15 is also coupled to a gas piston of a second stage (not shown). In this manner one gas piston draws in product gas as the other gas piston compresses product gas.
In an effort to prevent contamination of the product gas by hydraulic fluid that leaks from the hydraulic bore 27, migrates along the piston rod 25 and enters the gas chamber 17 via the gas bore 29, conventional compressors contain a first wiper 31 mounted along the piston rod 25 adjacent the hydraulic chamber 13, and/or a second wiper 33 mounted along the piston rod 25 adjacent the gas chamber 17. As the piston rod 25 passes through either wiper, a substantial portion of hydraulic fluid is wiped (i.e., removed) from the piston rod 25. Conventional compressors further provide the gas piston 23 with a number of seals (not shown) coupled between the outer surface of the gas piston 23 and the inner surface of the gas chamber 17, and vent the backside of the gas piston 23 (i.e., the portion of the gas chamber 17 located between the hydraulic chamber 13 and the gas piston 23) to ambient air. Another conventional method for reducing product gas contamination is to flow product gas to the backside of the gas piston 23 in an attempt to prevent ambient air contaminants from entering the backside of the gas chamber 17, adhering to the gas chamber's 17 walls and then transferring to the product gas as the gas piston 23 moves toward the hydraulic chamber 13.
While these conventional techniques do reduce product gas contamination to some extent, product gas contamination by hydraulic fluid particles nonetheless persists. Such contamination is particularly problematic in the semiconductor device fabrication field wherein a trace amount of hydraulic fluid may destroy a semiconductor device valued at $100,000 or more. Accordingly, a need exists in the compressor field for an apparatus and method that effectively isolates product fluid (e.g., product gas) from compressor operating fluid (e.g., hydraulic fluid).